REGULATION OF AFLATOXIN B-1-METABOLIZING ALDEHYDE REDUCTASE AND GLUTATHIONE-S-TRANSFERASE BY CHEMOPROTECTORS

Ingestion of aflatoxin B-1 (AFB(1)) represents a major risk factor in the aetiology of human hepatocellular carcinoma. In the rat, the harmf ul effects of AFB(1) can be prevented by the administration of certain drugs which induce hepatic detoxification enzymes. We have previously shown that treatment of rats with the chemoprotector ethoxyquin (EQ) results in a marked increase in expression of the Alpha-class glutathi one S-transferase (GST) Yc(2) subunit which has high activity towards AFB(1)-8,9-epoxide [Hayes, Judah, McLellan, Kerr, Peacock and Neal (19 91) Biochem. J. 279, 385-398]. To allow an assessment of whether the i ncreased expression of GST Yc(2) represents a general adaptive resista nce mechanism to chemical stress, that is invoked by both chemoprotect ors and carcinogens, we have examined the effects of EQ, butylated hyd roxyanisole (BHA), butylated hydroxytoluene (BHT), phenobarbital (PB), AFB(1), 3-methylcholanthrene (3-MC) and clofibrate on the AFB(1)-glut athione-conjugating activity and the GST subunit levels in rat liver. In addition, the effect of these drugs on the hepatic levels of an ald ehyde reductase (AFB(1)-AR) that metabolizes the cytotoxic dialdehydic form of AFB(1) has been studied as this enzyme also appears to be imp ortant in chemoprotection. Administration of the antioxidants EQ, BHA or BHT, as well as PB, led to a marked increase in levels of the GST Y c(2) subunit in rat liver, and this increase coincided with a substant ial rise in the GST activity towards AFB(1)-8,9-epoxide; neither AFB(1 ), 3-MC nor clofibrate caused induction of Yc(2) or any of the GST sub units examined. Among the xenobiotics studied, EQ was found to be the most effective inducing agent for the Yc(2) subunit as well as Yc(1), Yb-1 and Yf. However, PB was equally as effective as EQ in increasing levels of the Ya-type subunits, although it was not found to be as pot ent an inducer of the other GST subunits, including Yc(2). In addition to induction of GST, EQ caused a substantial increase in the hepatic content of AFB(1)-AR. Both BHA and BHT were also able to induce this e nzyme but, by contrast, PB was found to be a poor inducer of AFB(1)-AR . AFB(1), 3-MC and clofibrate were unable to serve as inducers of this reductase. The presence of Alpha-class GST, including the Yc(2) subun it, was examined in various rat tissues. Constitutive expression of Yc (2), was found in the epididymis at levels comparable with that observ ed in the liver from EQ-treated rats. Epididymal expression of Yc(2) w as accompanied by a high GSH-conjugating activity towards AFB(1)-8,9-e poxide which was not found in any other organ from normal rats. The hi ghest levels of AFB(1)-AR were found in the kidney, followed by modera te levels in the liver and testis. Other extrahepatic organs such as h eart, lung, spleen, adrenal gland, epididymis, vas deferens, seminal v esicles and ovary contain much lower levels of AFB(1)-AR.